I still use IE only because most webcontent relies on ActiveX. Once Java gets fully utilized, this will allow me to switch. I use firefox on linux because it has a good pop up blocker. In the mean time on WinXP, Norton Internet Security blocks ALL ads and intrusions and allows me to surf w/ IE and feel secure.
I switched in July and only use IE when I have to (updating windows software etc.). I've used Opera for a number of years as an alternative browser but I like Firefox better. Just a personal preference.